DE19903619C1 - Powder metallurgical composite material, especially for high voltage vacuum switch contacts, comprises refractory solid solution or intermetallic phase grains embedded in a metal matrix - Google Patents

Abstract

A powder metallurgical composite material comprises refractory solid solution or intermetallic phase grains embedded in a low melting metal matrix. A powder metallurgically produced composite material comprises a metal matrix of \} 1200 deg C melting point containing embedded grains of refractory components in solid solution or intermetallic phase form. An Independent claim is also included for production of the above composite material, in which the refractory solid solution or intermetallic phase is produced by cooling a melt at greater than 100 K/min. Preferred Features: The matrix consists of Cu, Ag and/or Al and the refractory components are selected from group Vb (V, Nb, Ta) and VIb (Cr, Mo, W) metals and their nitrides, carbides, silicides and/or borides.

Description

The invention relates to powder-metallurgically produced composite materials, comprising a matrix in which a granular additive is embedded, which consists of at least two refractory components, which as mixed crystals or there are intermetallic phases. The invention further relates to a Processes for their production and their use as contact materials, preferably in electrical vacuum interrupters.

Vacuum contact pads form the heart of the switching chambers in electrical vacuum switches and, according to the state of the art, generally consist of an arc-resistant, granular component (refractory metals such as W, Mo or Cr), embedded in a low-melting metal matrix with high conductivity (e.g. Ag, Cu or their alloys). The properties of the contact materials are subject to high and sometimes conflicting requirements, such as

• - low material burn-off,
• - sufficient switching capacity,
• - low tendency to sweat,
• - low electrical resistance,
• - good dielectric strength (dielectric strength),
• - low breaking current.

For vacuum circuit breakers in the medium voltage range <12 kV to approx. 30 kV and higher have proven to be particularly useful for CuCr composites proven. CuCr materials have very good current interruption properties and good dielectric strength (dielectric reconsolidation). At the in  this required performance range is the small number of 10,000 switching cycles the erosion resistance of CuCr materials is sufficient.

In the low voltage range <1,000 V, the use of Vacuum contactors are becoming increasingly important. The used in this area Sagittarius have to withstand 1,000,000 or more switching cycles, and that Tear current should be as low as possible. To those used here As a result, vacuum materials have additional main requirements.

High-performance materials for this area are pure W / Cu, WC / Ag, WC / Cu Form or with other additives. The matrix component is particularly effective here Ag has a good current breaking behavior, while the high-melting component W or WC minimizes arcing under the influence of an arc.

For the remaining gap between 1,000 and 12,000 V, it is difficult to design contact materials that meet the constantly increasing requirements for switching chambers for vacuum contactors due to the opposite properties of the pure refractory components W and Cr:

• - With increasing tension the use of a pure one takes place Tungsten component their limitation by increased tendency to Electron emission. This is the refractory nature of tungsten (mp 3.410 ° C) to ascribe. This increases the dielectric strength in a vacuum weakened.
• - In the case of low voltages, the reverse is used Cr refractory component its limitation due to the defective Burn-off resistance, which is shown by the total burn-up rate at high Switching operations results.

It would now be conceivable to mix the two differently melting metal parts Cr and W a refractory component quasi synthetic  to set an optimal profile for the desired voltage range Melting point (i.e. for the switching property the erosion resistance) and Electron emission (i.e. dielectric strength) would result. Im switching electrical contact should reflect the negative properties of the previous Contact materials used (for Cr due to the low Melting point the high burn rate, at W due to the high Melting point the high electron emission or the low Voltage resistance) neutralize.

An attempt in this direction is described in EP-A-0 083 245, which u. a. a CuCrW alloy disclosed, which in a manner known per se powder metallurgical way by pressing the metal powder mixture as well Sintering in solid or liquid Cu phase is produced. Aim according to this document is the production of a fine-grained composite. This is supposed to be through the Formation of a complete solid solution of the refractory metals in one another Caused by the metals W and Cr crystallizing in a cubic system become.

In order to test the usability of this teaching, composites were made from CuCrW according to the information in the publication. After sintering in the liquid Cu phase, the W grains are found in their original shape and size coated with Cr. The W-Cr particles are embedded in the Cu matrix ( Fig. 1). A mixed crystal of W and Cr of the type postulated in the publication could not be detected in any case. This is not surprising from a metallurgical point of view, since at the melting temperature of 1,100 to 1,240 ° C (above Cu liquidus) that can be assumed for this process, no reaction of tungsten with chromium is to be expected.

X-ray fluorescence analyzes of the material Cu 71% / Cr 24% / W 5% produced in accordance with the publication showed that the tungsten was insoluble in the surrounding matrix of Cr and Cu up to the detection limit. The sum analysis over a Cr area of 10 × 14 µm ² shows pure Cr, ie W is below the detection limit of <0.1% ( FIG. 2). Conversely, no diffusion of Cr into the W particles could be detected: a point analysis shows pure W, ie Cr is below the detection limit of <0.1% ( Fig. 3). Thus a mutual penetration of the refractory metals Cr and W, ie mixed crystal formation, does not appear to be feasible in this way. There is no teaching in this document to achieve improved switching properties by mixing the refractory components Cr and W as intimately as possible and by using the different properties of the two pure components.

EP-A-0 668 599 similarly discloses a contact material made of CuCr an additional auxiliary component from the group tungsten, molybdenum, tantalum and Niobium, which is obtained by diffusion of the refractory components in the liquid copper phase and subsequent quenching as a fine-grain distribution of the arc-resistant Components in the Cu matrix is generated.

For a CuCrW material, a mutual diffusion of Cr and W, as well as an arc-resistant grain made of Cr and W. The invention essentially aims at a fine-grained distribution of the individual Refractory components in the metal matrix. The formation of mixed crystals or intermetallic phases of the refractory parts with each other will not described.

The special requirements for materials for vacuum contactors for use in the voltage range between 1,000 and 12,000 V are therefore Mixtures of Cr and W described in Cu matrix not solved.

It is therefore an object of the invention to include a composite material a low-melting, current-carrying matrix made of, for example, Cu or Ag and to provide a granular additive from refractory components that the mentioned requirements for vacuum circuit breakers and vacuum contactors  is sufficient, i.e. both high dielectric strength and thus low Electron emission, as well as excellent erosion resistance and thus for use in particular in the voltage range from 1,000 to 12,000 V. is suitable.

Another object of the invention is to provide a method for Manufacture of such composite materials, which is carried out in an economical manner can be.

Finally, the object of the invention is a composite material for use as a contact material, preferably as a switching contact for vacuum interrupters in Voltage range of 1,000-12,000 V to provide.

These tasks were solved by the surprising finding that a Material with advantageous properties is obtained when the refractory portion no longer consists of particles of one or more refractory components, but when mixed crystals or intermetallic phases from at least two Refractory components are present, these being preferred Embodiment have significantly different melting points. It is from the point of view of metal science, the formation of a α phase, consisting of the pure or highly concentrated refractory component, not always avoidable. It is crucial, however, that it is also used for Formation of mixed crystals or intermetallic phases of the used Refractory components come, which leads to significantly improved properties (e.g. low electron emission) of the composite material. Preferably should Compositions can be selected that support the formation of α-phases exclude.

According to the desired material properties are not, as in State of the art, by joint sintering, alloying additional components in the low melting matrix or through  Mixing of different high-melting powder components set, but are replaced by pre-alloyed refractory components (in the form of Mixed crystals or intermetallic phases) modified.

The invention thus relates to a powder metallurgy Composite material comprising a matrix of a metal with a Melting point of at most 1,200 ° C and one embedded in this matrix granular addition of at least two refractory components, which thereby is characterized in that the refractory components in the form of mixed crystals or intermetallic phases.

Preferred embodiments of the composite material of the invention are Subject matter of claims 2-8. Among them, one is particularly preferred Composite material in which a or a first group of refractory components a melting point of 1,500 to 2,400 ° C and a second or a second Group of refractory components has a melting point of over 2,400 ° C.

Furthermore, a method for producing the composite material mentioned provided, which is characterized in that one powdery mixture of at least two refractory components Heating converts to a mixed crystal or an intermetallic phase and that powder obtained therefrom by cooling and crushing powder metallurgical route with a matrix metal with a melting point of connects at most 1,200 ° C.

Preferred embodiments of the method are the subject of the claims 10 and 11.

Another object of the invention is the use of the above Composite material as an electrical contact material, preferably as a switch contact  for vacuum interrupters, especially in the voltage range from 1,000 to 12,000 V.

The attached drawing shows:

FIG. 1 is a photomicrograph of a Cu-Cr W composite according to EP-A-0083245;

Figure 2 is an X-ray fluorescence analysis sum of Cr from the Cr Cu W- composite according to EP-A-0083245.

Fig. 3 is an X-ray fluorescence analysis of point W of the Cu Cr W composite according to EP-A-0083245;

Fig. 4 is a micrograph of Cr W 70/30 mixed crystals of Example 1;

Fig. 5 is a micrograph of Cr W 70/30 mixed crystals with a dendritic structure of Example 1;

Figure 6 is an X-ray fluorescence analysis sum of Cr and W from CrW 70/30 mixed crystals of Example 1.

Fig. 7 is a distribution analysis for W from Cr W 70/30 mixed crystals of Example 1; the white dots indicate W, the large black spots are pores in the melt cake;

Figure 8 is an X-ray fluorescence analysis sum of Cr and W from CrW 70/30 mixed crystals with chromium-rich sub-structure of Example 1.

Fig. 9 is a photomicrograph of Cr W mixed crystals in Cu matrix from Example 2;

FIG. 10 is an X-ray fluorescence analysis of a sum Cr ₂ Ta intermetallic phase of Example 3;

Fig. 11 is an SEM photograph of Cr ₂ Ta grains in the Cr matrix of Example 3;

FIG. 12 is a micrograph of Cr WC 70 / 28.2 / 1.8 mixed carbide with a dendritic structure of Example 4;

Fig. 13 is a micrograph of Cr WC 61/28/11 mixed carbide of Example 4.

The invention will now be explained in detail below.

The powder metallurgy composite of the present invention comprises a matrix of a metal with a melting point of at most 1200 ° C, in which a granular addition of at least two refractory components is embedded, the refractory components mixed crystals or include intermetallic phases from each other.

Relatively low-melting ones are suitable as the matrix of the composite material Metals with good electrical conductivity, as they are usually used for Vacuum contact pads are used. Cu is preferred as matrix material, Ag or Al. Alloys of these metals can also be used without that the proportions are critical.

Examples of refractory components that are suitable for use in the Compounds of the invention are suitable, namely the metals of groups Vb V, Nb and Ta, and VIb of the periodic table, namely Cr, Mo and W. In addition to the Metals in elemental form can also be nitrides, carbides, silicides or borides these metals (hereinafter referred to as "hard materials") and mixtures thereof or mixtures of the hard materials with the metals. The Use of the hard materials mentioned can change the properties of the Composite material, such as its weight, positively affect. The metals Cr and W are preferred as refractory components.

The quantity ratio of the refractory metals or hard materials used is not critical as long as it is guaranteed that by heating these components a Mixed crystal or an intermetallic phase is obtained. Within that The quantitative ratios of the metals or hard materials in certain limits wide ranges fluctuate. It is also within the scope of the invention if the ratio is such that only partly mixed crystals or  intermetallic phases arise while an excess Metal component remains partially as a pure substance.

The refractory component is preferably at least 1, preferably at least 5, more preferably at least 10 and in particular at least 50% by weight from mixed crystals and intermetallic phases. The is particularly preferably Refractory portion to more than 90% and especially completely as a mixed crystal or intermetallic phase.

The term "mixed crystals" are homogeneous solid solutions To understand refractory metals or hard materials, their places in the crystal lattice the atoms of the different metals are occupied. The atoms forming hard materials with a small radius, can be placed on metal interstitial spaces Host grid be stored; see. Römpp Lexikon Chemie, 10th edition 1998, p. 2705.

"Intermetallic phases" are chemical compounds of two or more metallic elements whose structure is distinct from that of the metals differs. In addition to phases with a stoichiometric composition according to the existing valences, there are also those in which these exact composition just a special case in a broad Represents area of homogeneity. Specific examples of the intermetallic phases are the Laves phases, Hume Rothery phases and Zintl phases; see. Rompp Lexicon Chemie, 10th edition 1998, p. 1943.

The share of the refractory components in the total mass of the Composite is not particularly critical, but is typically 15 to 80, preferably 25 to 50 wt .-%. The proportion is accordingly Matrix metals usually 20 to 85, preferably 50 to 75 wt .-%.  

In a preferred embodiment, the composite of the invention contains at least one refractory component with a melting point in the relatively low range of 1,500 to 2,400 ° C and at least one second refractory component with a relatively high melting point in the range of over 2,400 ° C. Examples of refractory components with a The melting point in the former range are Cr and Nb, while examples of the usable refractory components with a melting point above 2,400 ° C Metals are Ta, Mo and W. Preferred metal with lower melting point is Cr, preferred metal with a higher melting point is W. Also in this The farm is the quantitative ratio of the refractory components preferably such that when heated at least to a significant extent Mixed crystal or an intermetallic phase is formed.

From a practical point of view, it is desirable to use the respective amounts to select the refractory components so that they can be used with acceptable equipment Effort can be melted together. Accordingly, mixtures are made 10 to 90, preferably 30 to 70% by weight of the lower melting Refractory component, e.g. B. chromium and 10 to 90, preferably 30-70 wt .-% of higher melting refractory component, e.g. B. Tungsten. Particularly suitable a mixture of about 70 wt .-% Cr and 30 wt .-% W. In particular the amounts of the refractory components are selected such that the higher melting refractory in combination with the lower melting refractory component completely with formation of mixed crystals or intermetallic phases.

As a first step in the production of the composite material according to the invention become completely or almost completely homogeneous mixed crystals or intermetallic phases generated in that at least two Refractory components as a powder mixed intimately and with a suitable Process that can generate a high energy density in the melting volume melted quickly, homogenized and without significant segregation  be cooled down quickly. The rate of cooling should be greater than 100 K / min, otherwise the polygonal structure tends to separate. The cooled melt cake is then comminuted in a suitable manner and a Mixed crystal powder sieved according to the required grain size.

The mixed crystal powder obtained is further processed by powder metallurgy in one of the following ways together with a low-melting, highly electrically conductive matrix metal to give the desired composite:

• - Solid phase sintering:
  A metal powder of the low-melting matrix and the mixed crystal powder obtained are mixed, pressed and sintered below the melting point of the matrix metal, preferably under a high vacuum.
• - Sintering in the liquid phase:
  The mixed crystal powder obtained is pressed and preferably impregnated with the molten matrix metal under high vacuum.

The shaped bodies obtained from the sintering process are, due to the preferred high vacuum treatment, low in gas compared to parts conventionally sintered under protective gas, ie they contain reduced residual gas fractions of O ₂ , N ₂ or H ₂ . These blanks are further processed by machining finishing in the form of disks, rings or the like into suitable contact pieces which are used in vacuum interrupters.

The composite of the present invention takes place as a contact material for example in vacuum interrupters. They are particularly suitable Composites of the invention for use in the stress range of 1,000 up to 12,000 V, and here it is again the composite materials from at least each a refractory component with a melting point in the range of 1,500 up to 2,400 ° C and at least one refractory component with a melting point of over 2,400 ° C, the mixed crystals or intermetallic phases of these  Refractory components with melting points in each of the above Have areas.

The following non-limiting examples illustrate some preferred ones Embodiments of the invention:

example 1

70% by weight of Cr metal powder (Cr: ≧ 99.8% by weight) and 30% by weight of W metal powder (W: ≧ 99.95% by weight) are mixed, pressed and under vacuum or protective gas melted. The melt is quenched and solidified as quickly as possible (cooling rate <100 K / min). The resulting mixed crystals of W and Cr are almost homogeneous in their composition and show only weak inhomogeneities with regard to the mutual distribution of the individual components. The solidified melt shows a uniformly formed polygonal grain structure ( FIG. 4). The metal grains W or Cr originally used have completely dissolved in the melt and can no longer be detected in the melt cake.

With slow cooling (cooling rate <100 K / min.) The uniform polygonal structure tends to separate, a dendritic substructure is formed ( Fig. 5).

By scanning electron microscopy, these results can be as follows to confirm:

The sum analysis over a larger area of 0.5 × 0.7 mm ² defines the weight ratio W / Cr as 70/30 according to the weight ( Fig. 6). A distribution analysis W in Cr shows an essentially uniform distribution of W in Cr ( FIG. 7). This area corresponds to that of a uniform polygonal structure as in FIG. 4.

A determined maximum deviation from this ideal composition is shown by the sum analysis over a small area 20 × 28 μm ² on a substructure designed as dendrite with Cr = 54 and W = 46% by weight ( FIG. 8). This area corresponds to a section of mixed crystal with a substructure as in FIG. 5.

The melt cake is then broken up and ground. The The mixed crystal powder obtained is sieved to ≦ 160 µm and in the solid phase sintered. For this purpose, 75% by weight of Cu metal powder and 25% by weight of that obtained CrW mixed crystal powder mixed, pressed and below the melting point sintered by copper under high vacuum.

Example 2

A CrW mixed crystal powder is prepared as described in Example 1, sieved to ≦ 160 μm and sintered in the liquid phase. For this purpose, the CrW mixed crystal powder obtained is pressed and impregnated with liquid copper under high vacuum in such a way that a shaped piece in the composition Cu 60% by weight Cr / W 40% by weight is obtained ( FIG. 9).

Example 3

65% by weight of Cr metal powder (Cr ≧ 99.8% by weight) and 35% by weight Tantalum metal powder (Ta ≧ 99.9 wt .-%) are pressed and under vacuum or Shielding gas melted.  

The analysis of the melt cake obtained shows polygonal, primarily precipitated grains of the intermetallic phase Cr ₂ Ta in the composition CrTa 37/63% by weight (corresponding to 67 at.% Cr and 33 at.% Ta) (cf. FIG. 10) . These are surrounded by a matrix of Cr metal ( Fig. 11). This test result follows from the preselected composition of the green body from the metal components Cr and Ta close to the eutectic composition with ≈ 34% by weight tantalum. (see Massalski et al., Binary Alloy Phase Diagrams, Second Edition Vol. 2, p. 1339).

A Ta-rich starting composition up to a composition with 63-66% by weight Ta increases the proportion of the Cr ₂ Ta phase at the expense of the Cr phase up to pure Cr ₂ -Ta.

The melt cake is, as stated in Example 1 or 2, crushed and with Cu or Ag using powder metallurgy to form a metal composite processed further.

Example 4

70% by weight of Cr metal powder and 30% by weight of WC powder are mixed, pressed and melted under vacuum or protective gas. The high-melting toilet completely dissolves in the Cr melt that forms first. The melt cake solidifies to a CrW mixed carbide with the nominal composition Cr 70 / W 28.2 / C 1.8% by weight. Depending on the conduct of the solidification and cooling process, the metallographic analysis shows local inhomogeneities in the form of smaller or larger dendrites ( FIG. 12). The higher melting refractory component WC has completely dissolved in the new mixed carbide.

The carbon content in the mixed carbide can be increased if the pure Cr metal is replaced by the relatively low-melting Cr ₃ C ₂ (mp. 1,850 ° C). This corresponds to the condition from claim 3. The melt cake then solidifies with a nominal composition of 61% by weight Cr, 28% by weight W and 11% by weight C ( FIG. 13). Here, too, the original carbides have completely dissolved in favor of a new mixed carbide.

The melt cake is, as stated in Example 1 or 2, crushed and with Cu or Ag using powder metallurgy to form a metal composite processed further.

In a similar but not restrictive manner, carbides such as VC, NbC, TaC, TiC, nitrides such as TiN and TaN, silicides such as Ta ₂ Si and V ₃ Si and borides such as TiB ₂ can be used instead of the refractory components used here.

Example 5

As stated in Example 4, further melt cakes are made in the Compositions CrNb 50/50% by weight and CrMo 70/30% by weight mixed, pressed, melted, then crushed, sieved and added to the respective Processed combined with Cu or Ag.

Claims (14)

1. Powder metallurgically produced composite material, comprising a matrix of a metal with a melting point of at most 1200 ° C and a granular additive embedded in this matrix of at least two refractory components, characterized in that the refractory components comprise mixed crystals or intermetallic phases from one another. 2. Composite material according to claim 1, characterized in that the proportion the refractory components 15-80, preferably 25-50 wt .-%, and the Proportion of the matrix 20-85, preferably 50-75 wt .-%, based on the Total mass of the composite material. 3. Composite material according to one of claims 1 to 2, characterized characterized in that a refractory component or a first group of the refractory components a melting point in the range from 1,500 to 2,400 ° C and a second or a second group of refractory components have a melting point Have 2,400 ° C. 4. Composite material according to one of claims 1 to 3, characterized characterized in that the higher melting refractory component in Complete connection with the lower melting refractory component in the form of the formation of mixed crystals or intermetallic phases dissolves. 5. Composite material according to one of claims 1 to 4, characterized characterized in that the matrix of at least one of the metals Cu, Ag and Al exists.   6. Composite material according to one of claims 1 to 5, characterized characterized in that the refractory components from metals of group V b, (V, Nb, Ta) and VIb (Cr, Mo, W) of the periodic table and their Nitrides, carbides, silicides, borides and mixtures thereof are selected. 7. Composite material according to one of claims 1 to 6, characterized characterized in that the lower melting refractory component in an amount of 10-90, preferably 30-70 wt .-% based on the All of the refractory components are present. 8. Composite material according to one of claims 1 to 7, characterized characterized in that a lower melting refractory component Cr and is a higher melting refractory component W. 9. A method for producing a composite material according to one of the Claims 1 to 8, characterized in that a powdered Mix at least two refractory components by heating with high energy density melts, homogenizes and without any noteworthy Segregation cools down again at a cooling rate of <100 K / min, whereby the mixture into a mixed crystal or an intermetallic Phase is converted and that you can do it by cooling and Crushed powder obtained using a powder metallurgical method Matrix metal with a melting point of at most 1200 ° C connects. 10. The method according to claim 9, characterized in that a mixture one or a first group of refractory components with a Melting point from 1,500 to 2,400 ° C and a second or a second Group of refractory components with a melting point of over 2,400 ° C starts. 11. The method according to claim 9 or 10, characterized in that the Manufactured under protective gas or high vacuum.   12. Use of a composite material according to one of claims 1 to 8 as electrical contact material. 13. Use of a composite material according to one of claims 1 to 8 as a switching contact for vacuum interrupters. 14. Use of a composite material according to claim 13 in Voltage range from 1,000 to 12,000 V.